When the grade is steep a great deal of work must be done by the locomotive, much steam is required, and the quantity of fuel burned is large in proportion. When the road is level fuel burns less rapidly, and when the train stops, still more slowly. At night the locomotive rests, fires are banked and combustion is very slow. This process so briefly and incompletely sketched, is more interesting as one examines it closer, and a locomotive seems almost living when one considers minutely its wonderful performance.
But interesting and instructive though the operation of the locomotive may be, it is not for its own sake that I have mentioned it. It is rather in order to point out a remarkable parallel between its operation and that of a human body. A parallel, indeed, between the operation of a complex inanimate engine of iron and steel, and a still more complex living engine of flesh and bone and blood; both obeying the law of the conservation of energy, as well as the other laws of physics and chemistry.
Consider now a human body as a living engine. That man is more than matter is, of course, conceded. But we here regard only the animal body, guided by the brain as its engineer. The day begins, as with the locomotive, by taking a store of fuel and water, namely, food and drink. Food is not, however, burned in the body in a confined receptacle, like coal in the fire box of an engine, but is digested, assimilated and distributed through the body by means of the circulating blood. And while some of it goes to repair bodily waste, becoming tissue, other portions are oxidized or burned to produce heat. Non-digestible parts of the food pass away from the body as refuse, like ashes from the fire box of the engine. That the body fat and muscular tissue are also burned, producing heat, is literally true. A hibernating animal keeps his body warm all winter by burning up his autumnal store of body fat. Even a well-fed body is constantly wearing away, or burning away, and hence requires constant repair. Thus we see two distinct functions for food, which should be carefully distinguished.
In the first place, as already indicated, food repairs waste and builds up the body. It makes blood, bone and muscular tissue. Herein we see a departure from the parallel with the steam engine. A locomotive is a machine which runs in a way determined by its builder. But it cannot grow nor repair wear and tear. It requires a whole machine shop plus skilful mechanics to do that. The body, on the other hand, not only runs like a complex mechanism when supplied with energy, but also builds itself up and repairs waste. We express this by saying that it possesses vital force or life, but in just what vital force consists is a matter of speculation and controversy. The raw material which is employed in this work of repairing and building up is found in the food. But not all food can be so utilized. Only those materials which contain nitrogen, the so-called proteids, as lean meat, the casein of milk and gluten of wheat, can be made use of in this most important work of growth and repair.
In the second place, food is the fuel of the body and is just as truly burned as is coal in a furnace. Moreover, the quantity of heat which a piece of meat or a slice of bread yields when burned in the body is just the same as if it had been burned in a stove. Complete combustion yields a definite amount of heat wherever and whatever may be the place and manner of burning. Any kind of food may serve as fuel for the body, but those which consist mainly of sugar, starch and fat, which contain no nitrogen and so cannot build up the body, are used chiefly as fuel. These fuel foods form the bulk of our daily ration, comparatively little being required for purposes of growth and repair.
We are hearing a good deal recently about alcohol as a food. When it is remembered that alcohol contains no nitrogen it will be seen that it cannot serve the first function of food, namely, the purpose of growth and repair. It can, however, serve as fuel food, for when taken into the body in small quantities it is assimilated and burned up, producing the same amount of heat as if burned in a lamp. In sickness this may be beneficial, at times when the body cannot assimilate other foods. But the injurious effects of alcohol upon the digestive and nervous systems are so important and far-reaching that its value as a fuel food sinks into insignificance in comparison.
The process of combustion or burning in the fire box of our locomotive consists, as has been said, in oxygen of the air uniting with the carbon and hydrogen of the coal, forming carbonic acid and water, and setting free a definite quantity of heat for every pound of fuel so burned. So, in exactly the same way, oxygen, which has been taken up by the blood from the air in the lungs, unites with carbon and hydrogen in the tissues of the body and forms carbonic acid and water, yielding precisely the same amount of heat as though the combustion had occurred in a furnace. This idea of food, that it is literally fuel, is a very suggestive one. And as fuels differ in the quantity of ash contained and the amount of heat produced, so food materials differ in the quantity of undigestible residue and in their heat-producing power.
Remembering the analogy of the steam engine, let us now inquire what becomes of the energy supplied to the body in the fuel foods eaten, and which is turned into heat by this process of combustion constantly going on.
1. A large amount of heat is constantly being expended in keeping the body warm. Like the locomotive, the body is warmer than the surrounding air, and is constantly losing heat to the atmosphere. Unlike the locomotive, however, the body has a nearly uniform temperature throughout, namely, 98 degrees Fahr. The delicate regulation of temperature which is automatically maintained in the animal body is one of the wonders of physiology.
2. A second portion of energy is required to do the mechanical work of the body. When a locomotive hauls a loaded train up grade, or steams up grade alone, it is doing work in proportion to the total weight and the height to which it is carried. So when a man walks up hill or climbs a ladder he is lifting his body against the force of gravity, and hence doing work. If his weight be 200 pounds he is doing twice as much work as though he weighed only 100 pounds. If a man weighing 150 pounds climbs Bunker Hill Monument (220 feet), 33,000 foot-pounds of work will then be done; and if he succeeds in making the ascent in one minute, he would be doing work at the rate of one full horse power for that minute. If he climbs a mountain two miles high in three hours and twelve minutes he would be doing work in so lifting his body at the rate of one quarter of a horse power. This is, of course, a faster rate of work than an average man could maintain. In all the functions of daily life the body is necessarily doing some mechanical work. Even dressing and eating require a certain expenditure of energy, and in ordinary business and manual labor the amount of mechanical work done is considerable. Moreover, a large amount of work is done by the heart in pumping the blood through the circulatory system, and by the chest in respiration. This, then, the internal and external work done, as in the locomotive, represents the second portion of energy derived from the food eaten.